Achilles tendon (AT) stiffness is an important property of both human locomotor performance and injury mechanics. Freehand 3-D ultrasound (3-DUS) is a promising method for measuring stiffness of the Achilles tendon, particularly the free AT (2–6 cm proximal to calcaneus), which is commonly injured. The aim of this study was to investigate the test–retest reliability of freehand 3-DUS in measuring free AT stiffness in humans. The free Achilles tendon length of healthy participants (n = 10) was scanned on the same day on two consecutive occasions (1 h apart) during rest and isometric plantar flexion contractions at 20%, 40% and 60% of maximum force. The slope of the force–elongation curve over these force levels represented individual stiffness (N/mm). Relative reliability was assessed using the intra-class correlation coefficient (ICC), and absolute reliability was estimated with the standard error of measurement (SEM) and smallest detectable change. Systematic bias in stiffness measures was explored by comparing test and retest distributions and Bland–Altman plots. The test–retest reliability of free AT stiffness measured using freehand 3-DUS was excellent [ICC = 0.994, 95% confidence interval [CI]: 0.978–0.999)]. The mean stiffness values at test (361.83 N/mm [170.77]) and retest (364.98 N/mm [168.57]) did not significantly differ (p = 0.72), and the smallest detectable change was 52.14 N/mm. The Bland–Altman plot indicated the absence of systematic bias (95% CI: –22.18 to 15.88). Freehand 3-DUS provides reliable and precise measures of tendon stiffness and can be used to detect small changes in free AT stiffness in response to load or tendon pathology.

跟腱 (AT) 刚度是人类运动性能和损伤力学的重要特性。徒手 3-D 超声 (3-DUS) 是测量跟腱刚度的一种很有前景的方法，尤其是游离 AT（跟骨近端 2-6 cm），这是常见的损伤。本研究的目的是调查手绘 3-DUS 在测量人体游离 AT 刚度方面的重测信度。在休息和以 20%、40% 和 60% 的最大力量进行等长跖屈收缩期间，在同一天连续两次（间隔 1 小时）扫描健康参与者（n = 10）的游离跟腱长度。力-伸长率曲线在这些力水平上的斜率代表个体刚度 (N/mm)。使用类内相关系数 (ICC) 评估相对可靠性，绝对可靠性是通过测量标准误差 (SEM) 和可检测到的最小变化来估计的。通过比较测试和再测试分布以及 Bland-Altman 图来探索刚度测量的系统偏差。使用手绘 3-DUS 测量的游离 AT 刚度的重测可靠性非常好 [ICC = 0.994，95% 置信区间 [CI]：0.978-0.999)]。测试 (361.83 N/mm [170.77]) 和重新测试 (364.98 N/mm [168.57]) 时的平均刚度值没有显着差异（95% 置信区间 [CI]：0.978–0.999)]。测试 (361.83 N/mm [170.77]) 和重新测试 (364.98 N/mm [168.57]) 时的平均刚度值没有显着差异（95% 置信区间 [CI]：0.978–0.999)]。测试 (361.83 N/mm [170.77]) 和重新测试 (364.98 N/mm [168.57]) 时的平均刚度值没有显着差异（p  = 0.72)，最小的可检测变化为 52.14 N/mm。Bland-Altman 图表明不存在系统偏倚（95% CI：–22.18 至 15.88）。Freehand 3-DUS 提供可靠和精确的肌腱刚度测量，可用于检测响应负载或肌腱病理的自由 AT 刚度的微小变化。